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Usain Bolt emphatically declared himself the world's fastest man on Sunday by defending the 100m title he won so spectacularly in Beijing four years ago.

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Bolt came away with an Olympic record of 9.63sec, the second-fastest time ever recorded, and a margin of victory of 0.12sec over countryman Yohan Blake, an enormous gap in a race of this magnitude. If he can follow this with a repeat gold in the 200m tomorrow, he will surely have won his case as the greatest sprinter we have ever seen.

Ahead of the 100m final, much was made of Bolt's poor start, particularly after he false-started in the last year's World Championships.

The reality is that Bolt has always been quite a good starter, with reaction times at least comparable to his main rivals. On Sunday, he reacted to the gun in 0.165sec (legal is 0.100sec, while the fastest recorded was 0.139sec).

That is faster than both Blake and Justin Gatlin (0.179sec and 0.178sec respectively). So Bolt was not cautious in response to the gun, as many had speculated he might be.

His problem, however, is the first 30m. This is where his height hinders him - it is difficult to get those long legs and arms "out the way" as he drives out of the blocks from the low start position.

For this reason, his rivals often get the jump on him. We saw exactly that on Sunday, as Blake and Gatlin to Bolt's left both found themselves with a lead up to 40m, even though they were slower out of the blocks.

However, from 40m onwards, once up and running, Bolt is peerless. Those long legs, a disadvantage out of the blocks, now become one of his key advantages. Once he is running tall, he can use those levers to eat up the ground, reach top speeds faster than any other man in history, and maintain that speed for longer.

A sprinter typically hits top speeds of about 42km/h at the 60m mark of the race, and then begins a gradual slowing down until the finish line. For Bolt, that top speed has been clocked at over 44km/h, and he is able to limit the decline in speed more than his competitors.

In Sunday's Olympic final, once he was level with Blake and Gatlin at 60m, there was only going to be one winner. To illustrate his height advantage, Bolt took 41 steps to run the race, compared with 46 for Blake and 42.5 for Gatlin.

It would be wrong, however, to say that Bolt is the world's fastest man because he is tall - there are thousands of men as tall as Bolt who will never run 9.63sec.

The fact is, Bolt is good despite his height, because for most runners this kind of height is too big a barrier to overcome at the start. Bolt can do it and is exceptional because for a tall man, he has exceptional coordination and the ability to generate enormous forces in fractions of a second when his feet are on the ground.

The key to sprinting is not actually to move the legs quickly - research has revealed the amazing truth that Bolt, you and me are all able to swing our legs at about the same speed.

The difference is that the best sprinters use their legs as pistons which drive down and apply massive forces to the ground to propel their bodies back into the air for each step.

Bolt's competitive advantage is those huge forces, applied in that fraction of a second, combined with his long strides.